The present invention relates in general to ringing voltage generators for telephone networks, and is particularly directed to a new and improved ringing voltage supply scheme that involves the subdivision of a ringing bus into multiple ringing bus segments, to which redundant or auxiliary ringing voltage generators are selectively connectable, so as to ensure delivery of a ringing voltage signal as needed, regardless of a failure of a ringing voltage generator and/or removal of a ringing voltage generator card from its associated channel bank.
Digital subscriber loop (DSL) channel banks often require the generation of an AC voltage as a ringing signal on a customer""s POTS (plain old telephone service) line. This ringing signal is customarily generated as a periodic AC voltage waveform (e.g., sinusoidal, trapezoidal, etc.), having a relatively low frequency (for example, 16.5 Hz, 20 Hz, 25 Hz, 50 Hz are typical ringing frequencies). As a non-limiting example, the ringing voltage may comprise a sinusoidal waveform having an amplitude in a range on the order of from 40 to 120 Vrms riding on a DC voltage in a range on the order of from xe2x88x9230 to xe2x88x9270 VDC. In order to reduce amplitude variations for varying line/load conditions, it is generally desirable to employ closed loop regulation of the ringing voltage. In addition, it is also desirable to provide ringing voltage generation redundancy, in order to keep the telephone circuit operative in the event of a failure of a ringing generator module/card. Due to the statistical nature of the demand for ringing signal generation, it is further advantageous to have the full output power capability of any given ringing voltage generator available to meet peak demands.
Closed loop regulation of a sinusoidal (ringing) voltage that is available from multiple parallel-connected sources can involve the sharing of output voltage, feedback signals, reference voltages, and a reference (pulse width modulated (PWM)) clock. For true redundant or back-up operation, it is necessary to be able to isolate any ringing voltage generator in the event of a failure, since the failure of a generator connected directly in parallel with one or more other generators will disable every other unit until the failed generator can be identified and isolated or removed from the system. This can be a particularly cumbersome task, since identification of the faulty unit typically involves isolating and testing each generator on a one-by-one basis.
In accordance with the present invention, the desire to provide redundant telephone ringing voltage generator capability, without the shortcomings described above, is successfully addressed by subdividing the ringing voltage bus into multiple bus segments, and providing a plurality of redundant or auxiliary ringing voltage generators, that are individually and selectively connectable to multiple ringing bus segments. In the event of a failure of a ringing voltage generator, that faulty generator is automatically disconnected from its associated ringing bus segment, and that ringing bus segment is connected to a redundant ringing voltage generator.
In a first embodiment, a controlled ringing voltage source of each ringing generator is coupled to a failure detector and relay control circuit and to a relay circuit, which is configured to normally interconnect first and second ringing bus segments, and to isolate the ringing voltage generator from the ringing bus in the event of a failure. Only when energized does the relay couple the ringing voltage produced by the ringing voltage generator to its associated bus segment. Thus, if the relay control circuit detects an acceptable ringing voltage waveform from the ringing voltage generator, it energizes the relay, to isolate the two bus segments and couple the ringing voltage to its associated bus segment. However, if the failure detector and relay control circuit detects a faulty ringing voltage waveform from the ringing voltage generator, it maintains the state of its relay, so that the faulty ringing voltage is isolated, and both ringing bus segments are shorted together, to be supplied with a ringing voltage from the other xe2x80x98redundantxe2x80x99 ringing generator. Thus, as long as both ringing voltage generators are operating within normal parameters, each will supply a ringing voltage to its associated bus segment. However, during a failure, each voltage generator unit de-energizes its relay, so that its bus segment may be driven by the ringing generator associated with the other bus segment.
Although the first embodiment provides ringing voltage generation and fault isolation, it will not maintain the bus segments connected if either ringing generator module/card is physically removed from its channel bank card slot. To obviate this potential problem, switching path connections to the bus segments may be modified to include additional cross-coupled bus-shorting switching devices that are respectively controlled by the other ringing generator module. As in the first embodiment, if the failure detector and relay control circuit determines that the ringing voltage waveform is defective, it keeps its ringing voltage coupling relay de-energized, so that the defective ringing voltage generator is isolated from the ringing bus. In addition, it allows the bus-shorting relay on the other module to maintain the two bus segments coupled together, so that they may both receive ringing voltage from the other redundant generator.
In accordance with a third embodiment of the invention, auxiliary signals associated with the ringing voltage, such as zero-crossing signals, status signals, synchronization signals and the like, have their signaling paths segmented in association with the segmentation of the ringing voltage leads. For the case of zero-crossing signals, as a non-limiting example, zero-crossing detectors are provided to minimize noise transients and stress on downstream ringing voltage switching devices connected to the ringing bus segments. Each relay is configured as a double-pole relay to couple an additional zero-crossing detection signal to an associated zero-crossing line; the relay control circuit has a zero-crossing detection output that changes state at the time of occurrence of a zero-crossing in the monitored ringing voltage waveform.
As in the second embodiment, if the failure detector and relay control circuit determines that the monitored parameters of the ringing voltage waveform are acceptable, it operates an associated relay to couple the ringing generator to its associated bus segment, and to couple the zero-crossing output to the zero-crossing line associated with the ringing voltage generated by the ringing generator. However, for a faulty ringing voltage waveform, it will maintain the relay de-energized, so that the ringing bus segment is isolated from the ringing voltage generator and the zero-crossing output is isolated from the zero-crossing line.
In addition to the first double-pole relay, a respective ringing generator module of the third embodiment includes a second double-pole, bus-shorting relay coupled to the segmented ringing bus, and the segmented zero-crossing bus. This bus-shorting relay is controlled by an inverted output of the failure detector and relay control circuit of the other ringing generator module. This xe2x80x98inverter-coupledxe2x80x99 circuit configuration performs the same relay control functionality as the second embodiment; however, it operates only one relay at any time, and thereby reduces the amount of drive current required by the control signal and reduces power wasted in the relay during normal operation. The use of an inverter ensures that the contacts of each relay will be open at the time of insertion of the circuit card containing those relays into a channel bank, which eliminates surge currents and noise transients during card insertion.
The ringing voltage and zero-crossing redundancy functionality of the present invention may be extended to any number of ringing bus segments and associated ringing generator modules. In such extended configurations, the ringing bus segments are connected to their associated ringing generator modules in a daisy chain architecture so that failure of one or more ringing generator modules will short their associated bus segments and zero-crossing lines to a remaining one or more of the respective bus segments and zero-crossing lines.
In addition to the use of internal (card-installed) ringing voltage generators, the ringing voltage generator for one of the ringing generator modules may be external to the module, to accommodate the option of using internal ring generators (on plug-in cards) and/or external ring generators (large system generators). In a first xe2x80x98external optionxe2x80x99 embodiment, the external ringing voltage is routed through the channel bank into the module which supplies a selected bus segment. Should the externally supplied ringing generator module fail, the internal ringing generator on the other module will supply a ringing voltage to both bus segments. If the internal ringing voltage generator within the other ringing generator module fails, the externally supplied ringing voltage will provide a ringing voltage to both bus segments.
The external option module can either replace the internal ringing voltage generator-based module or the module may employ both on-board ringing voltage generation and an external ringing voltage feed. With both internally sourced and externally supplied ringing voltage feeds on a single card, the user has the choice of which may is to used for powering the single bus (via a jumper or relay). In accordance with a further, external option embodiment, each ringing generator module includes a separate relay path, and an associated failure detector and relay control circuit for applying the externally supplied ringing voltage to the bus segment associated with that module. Each ringing generator has an external ringing voltage supply port is coupled to an external voltage failure detector and relay control circuit and to an external ringing voltage relay circuit, that is operative to controllably isolate the external ringing voltage port from its associated ringing bus segment in the absence of a failure. Only when this relay is energized in response to a failure of the internal voltage generator does the relay couple the external ringing voltage to its associated bus segment.
Should both the external ringing voltage and the internal ringing voltage of one module fail, the other module will be triggered to interconnect both bus segments and supply either its internal or externally supplied ringing voltage to the bus. Because of this dual redundancy, all four ringing voltage generators (two internal and two external) must fail before the channel bank loses ringing capability. If only one external ringing voltage source is available, the external voltage supply ports may be connected together by way of a jumper wire and the like. Changing the order of which module takes over for a failed ringing voltage source is readily effected by rearranging the logic circuits and relays.